As materials of optical substrates such as spectacle lenses, plastic substrates have been increasingly used instead of inorganic glass substrates in recent years. This is because the plastic substrates have excellent properties in respects of lightweight properties, impact resistance, processability, dyeing properties, and the like. The plastic substrates, however, have a defect that they are liable to be damaged as compared with the inorganic glass substrates.
In order to avoid this defect, a silicone-based coating film, namely, a film of a hard coat layer, is usually provided on a surface of an optical lens which is a plastic substrate. Moreover, when a plastic substrate with a relatively high refractive index is used as a material of an optical lens, such a treatment that metal oxide particles are incorporated into the film of the hard coat layer to adjust a refractive index of the film of the hard coat layer to a refractive index of the plastic substrate has been carried out in order to avoid interference of light (appearing as interference fringes) occurring between the plastic substrate and the film of the hard coat layer.
As for a coating liquid for forming a silicone-based coating film having such properties on a plastic substrate as, for example, a film of a hard coat layer, various developments have been made, and a large number of applications have been made.
In the production of optical substrates, in particular plastic lens substrates such as spectacle lenses, there have been desires for a coating liquid for forming a coating film with a high refractive index and transparent colorless, which is excellent in properties, such as scratch resistance, abrasion resistance, impact resistance, weathering resistance, light resistance, perspiration resistance, hot water resistance, adhesion properties and dyeing properties, and there have been also desires for a sol of a water dispersion or a sol of an organic solvent containing metal oxide particles, which is a composition as a raw material of the coating liquid. A large number of applications relating to those also have been made so far.
For example, Patent literature 1 discloses a coating composition with a high-refractive index, which contains: metal oxide particles containing at least one selected from silica, iron oxide, titanium oxide, cerium oxide, zirconium oxide, antimony oxide, zinc oxide and tin oxide, or a mixture thereof, or metal oxide particles constituted with a composite oxide of the metal oxide as described above; and an organosilicon compound. A coating film formed by use of a coating liquid containing these metal oxide particles has a relatively high refractive index, but is far from excellent weather resistance.
This has a background as follows. In the optical substrates such as spectacle lenses, the thickness of a plastic lens or the like has been decreased in order to meet a demand for weight saving. Decrease of the thickness has prompted the refractive index of the coating film to be higher. As a result, the content of titanium oxide with high refractive index properties tends to be increased. On the other hand, the titanium oxide has a photocatalytic activity and therefore has impaired weathering resistance of the coating film.
Then, the present inventors have developed a dispersion sol containing fine particles constituted with core particles containing a titanium-based oxide and a coating layer formed on a surface each of the core particles containing a titanium-based oxide with a composite oxide of silicon and zirconium and/or aluminum, and have also developed a coating liquid for forming a coating film containing the fine particles and an organosilicon compound. The present inventors have applied for patent on the dispersion sol and the coating liquid. In detail, by coating the core particles containing a titanium-based oxide with the composite oxide, photocatalytic activity of the titanium oxide contained in the core particles is restrained.
For example, Patent literature 2 discloses a dispersion sol containing (1) fine particles constituted with fine particles (as core particles) of titanium oxide and a coating layer formed on a surface each of the fine particles of titanium oxide with zirconium oxide and silicon oxide, (2) fine particles constituted with fine particles (as core particles) of composite oxide made from a solid solution of titanium oxide and zirconium oxide and a coating layer formed on a surface each of the fine particles of composite oxide with silicon oxide, (3) fine particles constituted with fine particles (as core particles) of composite oxide of titanium and silicon and a coating layer formed on a surface each of the fine particles of composite oxide of titanium and silicon with silicon oxide and zirconium oxide and/or aluminum oxide, or (4) fine particles constituted with fine particles (as core particles) of composite oxide of titanium, silicon and zirconium and a coating layer formed on a surface each of the fine particles of composite oxide of titanium, silicon and zirconium with at least one of silicon oxide, zirconium oxide and aluminum oxide. Patent literature 2 also discloses a coating liquid for forming a coating film containing the above fine particles and an organosilicon compound. In detail, the invention disclosed in Patent literature 2 involves use of metal oxide particles with a core-shell structure constituted with titanium-containing core particles with an anatase-type crystal structure and a coating layer formed on a surface each of the titanium-containing core particles with at least one selected from silicon oxide, zirconium oxide and aluminum oxide.
Patent literature 3 discloses a dispersion sol containing fine particles of composite oxide constituted with a composite oxide (as core particles) of solid solution of titanium and tin and a coating layer formed on a surface each of composite oxide (as core particles) of solid solution of titanium and tin with a composite oxide of silicon oxide and an oxide of zirconium and/or aluminum. Patent literature 3 also discloses a coating liquid for forming a coating film containing the above fine particles and an organosilicon compound. In detail, the invention disclosed in Patent literature 3 involves use of metal oxide particles with a core-shell structure constituted with titanium-containing core particles with a rutile-type crystal structure and a coating layer formed on a surface each of titanium-containing core particles with a composite oxide of silicon oxide and an oxide of zirconium and/or aluminum.
Use of the metal oxide particles described in Patent literature 2 or Patent literature 3 allows a coating film to be given with not only excellent weathering resistance but also excellent other properties, such as scratch resistance, abrasion resistance, impact resistance, light resistance, perspiration resistance, hot water resistance, adhesion properties, transparency and dyeing properties, provided that the refractive index is in the range of 1.52 to 1.67.
However, an optical substrate (in particular, a plastic lens substrate) with a refractive index of 1.70 or more, more particularly 1.71 to 1.81, has been developed recently, and there have been demands for a coating liquid for forming a coating film appropriate for this optical substrate and a composition as a raw material (i.e., a dispersion sol containing metal oxide particles) for the coating liquid. In order to raise the refractive index of the coating film, it is necessary to further increase the titanium content in the core particles or further decrease the thickness of the coating layers (i.e., coating layers on the surfaces of the titanium-containing core particles). As a result, a coating film with a relatively high refractive index of about 1.70 has been obtained, but the weathering resistance and light resistance thereof tend to be impaired. Moreover, it is difficult to obtain a coating film with a refractive index higher than about 1.70.
In addition, a defect of inferior impact resistance arises in the case of an optical lens such as a spectacle lens, which is obtained by forming the aforesaid film of the hard coat layer on a surface of a plastic substrate and further forming an antireflection film thereon.
As means to overcome this defect, there are known methods such as (1) a method of forming a film of a primer layer containing a thermosetting urethane resin and metal oxide particles containing titanium oxide (e.g., as shown in Patent literature 4), (2) a method of forming a film of a primer layer containing a polyurethane resin and metal oxide particles of zinc oxide, silicon dioxide, aluminum oxide, titanium oxide, zirconium oxide, tin oxide, beryllium oxide, antimony oxide, tungsten oxide, cerium oxide or the like (e.g., as shown in Patent literature 5). In these methods, the metal oxide particles are added in order to control refractive index of the coating film (restraint of interference of light) or in order to enhance strength of the coating film. If the titanium content in the fine particles is increased for the purpose of making the refractive index of the optical lens higher, there occurs a problem that the weathering resistance and the light resistance of the coating film are deteriorated, similarly to the case of the aforesaid film of the hard coat layer.